1. Field
This patent specification relates to an overlay target and a method for determining the precision of mask alignment for optical lithography during semiconductor device fabrication processes or other projection printing.
2. Description
As semiconductor devices become more miniaturized, the demand for registration accuracy in photolithography is ever increasing.
The accuracy of mask alignment has been measured typically by patterning a resist pattern over a substrate pattern, measuring the positions both of the overlaid resist pattern and substrate pattern with an optical positioning measurement equipment and determining an amount of displacement of these patterns.
As an example of prior measurement methods a box-in-box method has been used, which is described hereinbelow, referring to FIGS. 1A through 1C.
FIG. 1A is a top view illustrating two boxes (or box marks) forming a box-in-box overlay target and FIG. 1B is a cross-sectional view taken along the line A-A' of the structure of FIGS. 1A. For use of a position measurement, a substrate box 3 is formed as a concave square on a substrate 1. An underlayer 5 of silicon oxide and/or other similar materials is subsequently formed over the substrate box 3 and the substrate 1. Furthermore, another box 7 of a photo-resist material is formed over the underlayer 5, having its sides shorter than those of box 3, to thereby be included in the substrate box 3 when viewed from above, as illustrated in FIG. 1A.
The overlay target for the precision measurement is therefore composed of inner and outer boxes 7 and 3, each having a side length of approximately from 10 to 40 microns and from 20 to 50 microns, respectively.
Using these box marks, mask alignment is measured as follows. (1) The positions of edge portions are observed for the substrate box 3 and top photo-resist box 7 with an optical positioning measurement equipment (i.e., overlay registration measurement system) within dotted areas 9 (FIG. 1C). (2) The dotted areas 9 of the substrate box mark 3 and resist box mark 7 are measured by scanning, and image data for each area 9 are inputted and then processed pixel by pixel to measure the position of the edge portions. By repeating this procedure for all of the eight dotted areas 9 of the substrate box 3 and resist box mark 7, the positions thereof are determined. (3) From the results obtained as above, center positions are determined for each of the substrate box mark 3 and top box mark 7, and a positional deviation between these two centers is determined.
However, this method has a disadvantage in that it takes a relatively long time to determine the positional deviation due to the repetition of measurements such as, for example, eight times in the above case.
Further, if the thickness of the underlayer 5a (FIG. 2B) is different from one edge portion to another on a substrate box 3a, the number of pixels counted for one edge portion may differ from that for another edge portion even when the box mark's centers coincide. Since the pixel number is taken to be related to the dimension during image signal processing, the difference in the pixel number mentioned above can result in reduced precision.
Still further, light may be incident on the box marks during optical measurements with its beam axis 10 not in parallel to a surface of the side edge of the resist box mark 7 (FIG. 2A) , thereby forming a shadow on the face on the layer 5. This shadow may be recognized and then inputted as a part of the resist mask 7 by an overlay registration measurement system to possibly result in erroneous length values, which is known as a tool-induced shift (TIS) by optical measurement system in prior measurement methods. In addition, since beam focusing during the measurements is intended simultaneously for edges of both the substrate box mark 3 and resist box mark 7, it becomes difficult or sometimes not feasible to achieve contrasts satisfactory enough to recognize images, thereby possibly leading to a reduction in the reproducibility of measurements.